Polyolefin resins have advantages in their excellent moldability, heat resistance, mechanical properties, low specific gravities and the like, and have been widely used for films, sheets and various molded articles (e.g., structural parts). However, since polyolefin resins are slow in crystallization rate after molding, they are poor in the molding cycle performance, and have drawbacks of insufficient transparency and strength because of the generation of large crystals depending on crystallization progress after heat molding.
It has been known that all of these drawbacks are due to crystallinity of polyolefin resins and are resolved by rising crystallization temperature and rapidly generating fine crystals.
It is common knowledge that a crystalline nucleating agent or a crystallization accelerator is added in order to achieve this purpose. As the crystalline nucleating agent or the crystallization accelerator, compounds such as carboxylic acid metal salts such as sodium benzoate, aluminum 4-tert-butylbenzoate, sodium adipate and disodium bicyclo[2.2.1]heptane-2,3-dicarboxylate; phosphoric acid ester metal salts such as sodium bis(4-tert-butylphenyl)phosphate, sodium 2,2′-methylenebis(4,6-di-tert-butylphenyl)phosphate and lithium 2,2′-methylenebis(4,6-di-tert-butylphenyl)phosphate; and derivatives of polyols such as dibenzylidene sorbitol, bis(methylbenzylidene)sorbitol and bis(dimethylbenzylidene) sorbitol have been used conventionally.
Among these compounds, as a compound having especially great improving effect on transparency of polyolefin resin, for example, compounds of dibenzylidene sorbitol are proposed in Patent Documents 1 and 2 and the like, and dibenzylidene sorbitol compounds of which the benzene ring has substitution group are proposed in Patent Documents 3 to 5 and the like. Also, acetal compounds having substituted aryl group are proposed in Patent Document 6.
In addition, a cyclic phosphoric acid ester metal salt of alkylidene bisphenols is proposed in Patent Documents 7 to 10 and the like.
Furthermore, several approaches have been attempted to improve the effect by combining these compounds. For example, a combination use of a phosphate compound and a dibenzylidene sorbitol compound to crystalline propylene polymer is proposed in Patent Documents 11 and 12, and a combination use of a dibenzylidene sorbitol derivative and an aromatic organic phosphate ester metal salt to crystalline synthetic resin is proposed in Patent Document 13.
Benzylidene sorbitol compounds have been used as a crystalline nucleating agent capable of greatly improving optical properties, in particular, transparency of molded article produced by processing a polyolefin resin, while in the field that needs higher transparency, they cannot sufficiently satisfy the requirement.
As a method to overcome such problems, for example, a combination use of a cyclic phosphoric acid ester metal salt and bis(4-alkylbenzylidene)sorbitol to crystalline propylene polymer is proposed in Patent Document 11, a combination use of an aromatic phosphoric acid ester metal salt and a dibenzylidene sorbitol compound to crystalline propylene polymer is proposed in Patent Document 12, and a combination use of a dibenzylidene sorbitol derivative and an aromatic organic phosphoric acid ester metal salt to crystalline synthetic resin is proposed in Patent Document 13.